3GPP-LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme. According to 3GPP-LTE, a radio communication base station apparatus (hereinafter abbreviated as “base station”) transmits reference signals (RSs) using predetermined communication resources and a radio communication terminal apparatus (hereinafter abbreviated as “terminal”) performs channel estimation using the received reference signals and demodulates data (see non-patent literature 1). Furthermore, using reference signals, the terminal performs measurement of receiving quality for adaptive MCS (Modulation and channel Coding Scheme) control, for PMI (Precoding Matrix Indicator) control in MIMO (Multiple-Input Multiple-Output) transmission or for adaptive scheduling. The terminal then feeds back the obtained PMI and receiving quality information (CQI: Channel Quality Indicator) to the base station.
Furthermore, when the base station is provided with a plurality of antennas, the base station can perform diversity transmission. For example, the base station can realize high-speed transmission by transmitting a plurality of data streams from a plurality of antennas (MIMO transmission). In order for the terminal to receive the signal transmitted with diversity without errors, the terminal has to know a channel condition from a plurality of antennas used for transmission by the base station to the terminal. Therefore, RSs need to be transmitted from all antennas provided for the base station without interference with each other. To realize this, 3GPP-LTE employs a method of transmitting RS from each antenna of the base station using timings and carrier frequencies different from each other in the time domain and frequency domain.
FIG. 1 shows a configuration of a 4-antenna base station (4Tx base station) envisioned by 3GPP-LTE and FIG. 2 shows an RS transmission method by the 4Tx base station (see non-patent literature 2). Here, in FIG. 2, the vertical axis (frequency domain) corresponds to a subcarrier unit and the horizontal axis (time domain) corresponds to an OFDM symbol unit. Furthermore, R0, R1, R2 and R3 represent RSs transmitted from antennas 0, 1, 2 and 3 (first, second, third and fourth antennas) respectively. Furthermore, in FIG. 2, a unit of one block enclosed by a thick line frame (six subcarriers in the frequency domain and fourteen OFDM symbols in the time domain) is called “resource block (RB).” Though one RB is comprised of 12 subcarriers according to 3GPP-LTE, it is assumed here that the number of subcarriers, of which one RB is comprised, is six for ease of explanation. Furthermore, a unit of 1 subcarrier×1 OFDM symbol, of which one RB is comprised, is called “resources element (RE).” As is clear from FIG. 2, the 4Tx base station reduces transmission frequencies of RSs (R2 and R3) from antenna 2 (third antenna) and antenna 3 (fourth antenna) to minimize overhead involved in RS transmission.
The RSs shown in FIG. 2 are common to all terminals in a cell covered by the base station and are called “cell-specific RSs (cell-specific reference signals).” Furthermore, the base station may also additionally transmit RSs (terminal-specific RSs (UE specific reference signals)) multiplied by a weight specific to each terminal for beam forming transmission.
As described above, the number of antennas of a base station according to 3GPP-LTE is a maximum of four and a 3GPP-LTE-compliant terminal demodulates data and measures quality of a downlink signal using RSs (R0 to R3 shown in FIG. 2) transmitted from a base station (4Tx base station) provided with a maximum of four antennas.
By contrast, LTE-advanced which is an evolved version of 3GPP-LTE is studying a base station equipped with a maximum of 8 antennas (8Tx base station). However, LTE-advanced is also required to provide a 3GPP-LTE-compliant base station to enable terminals compliant with only a 3GPP-LTE base station (4Tx base station) to communicate. In other words, LTE-advanced is required to accommodate both terminals compliant with only a 4Tx base station (hereinafter referred to as “LTE terminals”) and terminals also compliant with an 8Tx base station (hereinafter referred to as “LTE+ terminals”).